1. Field of the Invention
This invention concerns a network system transmission control method by which multiple transmission stations mutually exchange data.
2. Description of the Related Art
Multiple control systems are positioned dispersed on the production lines of industrial facilities and the like, and network systems are used to exchange data so that various types of equipment are correlated and operate in cooperation. As control systems, there are such systems as programmable controllers which control various types of production machinery, measuring systems, I/O (input/output) systems, central operator operated monitoring systems which monitor the state of a line and execute operations, and control computers which execute production quality control and production result control, production program control, and the like. There are many configurations for network systems such as bus type, star type, ring type and also mesh type.
FIG. 20 shows a bus type network system as one example of this type of system. It is composed by connecting multiple transmission stations (STN #n) which execute control of data transfer mutually between control systems to a common transmission path (L).
Each control system (not shown in the drawing) is connected to any of the transmission stations. When the transmission and reception of data is executed by each control system, the control system generally executes a data transfer request by adding data attributes, such as transfer addressee and transmission originator information, information on the data to be transmitted and processing information by the transfer addressee, to the exchange data. The transmission station which receives the data transfer request collates the transfer data into data packets and executes an output request. The output requests from multiple transmission stations are scheduled, and, after permission to use the common transmission path has been obtained, the station outputs the data packets to the transmission path. The data packets which are outputted to the transmission path are received by the transfer addressee transmission station which is designated in the address. The exchanged data and the data attributes are delivered to the transfer addressee control system. Thus, it is possible for the control systems to exchange data with each other.
Also, with this type of network system, because of such factors as simplicity of design by apportioning of transmission zones, real-time capability and data recoverability, normally, in a constant transmission cycle, each control system broadcasts its respective input and output data to all the other control systems. Through each control system receiving and inputting this, the respective control systems hold the input and output data of the other control systems in common, and a transmission system is used which enables data exchange between the control systems merely by reading from and writing to a memory.
FIG. 20 shows the outline of this type of transmission system. It shows the data in an allocated area (D3) in the common memory (CM) of transmission station STN#3 being broadcast, and being inputted by other transmission stations and being stored in the respective areas (D3) of common memory (CM). By each transmission station executing the broadcast of data in the same way, each control system can easily execute exchange of data with other control systems via the common memory (CM).
With this type of network system, the total volume of data held in common by the network system as a whole, that is to say between the control systems, is refreshed every transmission cycle, regardless of whether or not there is any variation in the data content, by mutual broadcast exchanges in constant transmission cycles. In other words, forming a virtual common memory in the network and taking transmission cycles as units, makes it easier to design good definition of the working times and the necessary transmission zones for data exchange. Recovery from the dropping of transitory data due to refreshing every transmission cycle also becomes possible. Also, it enables co-operative operation between various control systems based on status information for the whole of the control process which is developed in the virtual common memory.
In recent years, due to the rapid development of electronic technology and networking of its surrounding environment, there has been a tendency to make networking control systems more and more intelligent. As a result, the making of control systems practically autonomous and their dispersion have steadily developed, and their unitization can be considered. For communication between autonomous, dispersed control systems, it is considered that a network system with an infrastructure which can support a high degree of mutual co-operation is required.
The above network system makes possible simple communication between application processes via a common memory. However, conversely, the amount of data which can be exchanged between application processes via a common memory is limited by the balance between the time intervals which refresh the capacity of the common memory and the transfer speed which can be achieved at an appropriate cost.
In communications between unitized control systems as network applications hereafter, for example in the case of multimedia information such as images and voice being exchanged with the same degree of importance as control information, that is to say the communications data amount in response to an event which occurs unexpectedly, will vary dynamically depending on the state of the plant. Moreover, in the case of its being necessary to satisfy the required time restraints on control monitoring, with methods which take as prerequisites fixed data volumes and assignments as do the present network system methods, it is difficult to satisfy these fluctuating data communication requirements.
However, this does not mean that either a set-up which acts in cooperation by the control systems which are executed by present network systems holding in common the fixed input/output data which they mainly handle, that is to say by holding in common control state information, or a set-up which acts in cooperation by exchanging and holding in common event information, which corresponds to events, between respective application processes which are functional units and act autonomously is required. Their ratio will alter depending on the application. It is considered that a network system will become necessary which can support in a consistent form the data communications which both set-ups make necessary. In this case, as can be seen in present network systems, it is difficult to handle both functions with consistency in systems which take the method of fixed apportionment of transmission zones as a basis.
Also, with prior art network systems, in the case of a control system attempting data communication in response to an event which occurs unexpectedly, there is the problem that the output of data packets to the network system is not always at the expected time, and outputted data packets are not necessarily transferred to the final intended address within the expected time, affected in the state of generation of data transmission requests from one moment to the next over the entire network.
The cause of this is that, in order for the applications process group in a network system to hold in common and make common use of resources which participate in the communications of a network while being mutually interrelated, when data packet transfer requests accumulate, the stagnation of waiting for the allocation of communications resources in the network system occurs.
Therefore, consideration has been given to attempts to allocate resources so that the data packet transfer requests requested by the application processes group do not exceed the processing capability possessed by the communications resources and accumulate, and to regulate the timing and sequence of the generation of data packet transfer requests so that they do not build up excessively.
For example, with ATM (Asynchronous Transfer Mode), the design is that when there is a request for data communication from a host, the data amount which can be outputted by the host per unit time is determined by negotiation. Also, in the case of a host contravening this and trying to pass an oversized cell into the network for example, stagnation of the network is prevented by discarding the oversized cell as a deviant cell. As a result, the design is to increase the utilization efficiency for the many host machines which are connected to the network.
However, with a system like ATM which controls stagnation by determining in advance the communications capacity which can be used by negotiation, and each host machine observing this, in other words a system which executes data transfer within a communications capability determined by negotiation, there is a requirement to execute the setting of negotiation rules by using mean values and peak values of communications volume. As a result, it becomes a fixed network system.
With data communication between control systems in the future, the generation of communications requests will become increasingly random, and it will be difficult to make a fixed system, in which the communications capability is determined in advance, respond efficiently. Also, in relation to advance stipulation so that the generation timings and sequence of data packet transfer requests do not excessively accumulate, because transfer requests are random due to the irregularity attendant upon the working state, it is difficult correctly to regulate and control that generation timing and sequence beforehand.
For instance, even if, by some means or other, this is once precisely regulated, in its actual application, application process groups will repeatedly actively participate in the network system and separate from it. Moreover, when new application processes are added, application processes which have hitherto been operating are adjusted or stopped, and the network is expanded or adjusted, the sequence will soon lose its meaning. That is to say, when considering the actual state, the regulation of the sequence of data packets exchanged by the application processes group is impossible in itself.
Therefore, as a concept for the unified handling of the above, the present invention considers it satisfactory if the data exchanged between application processes which work in cooperation is correctly delivered to the addressee within the limit time required by an application. Taking this limit time, that is to say, as a basis the data transfer completion time to the addressee application processes, the present inventors have proposed methods (Laid-Open Patent Heisei 6-6407, Laid-Open Patent Heisei 6-152604 and Laid-Open Patent Heisei 8-32623) for the execution of processing concerning data transfer by taking it that the shortest transfer completion time for a data transfer request generated within a network system is the most urgent.